In various systems, TTT (Time to Trigger) is used to adjust a lag time from when a condition for state transition from one operating state to another operating state is satisfied to when the state transition is actually initiated. TTT is also called hold time or guard time in some cases. In general, TTT is used applied to determination of a period during which a state transition condition need to be kept satisfied. Specifically, when a state transition condition continues to be satisfied for a longer time than TTT, state transition is actually initiated. Therefore, a state transition is less likely to occur as TTT is longer. By adopting hysteresis using TTT, it is possible to prevent ping-pong phenomenon where state transitions between two operating states repeat frequently.
In cellular radio communications, TTT is used as one of handover control parameters (which are referred to hereinafter as handover (HO) parameters) in order to prevent handover failure of a mobile station, repetition of handover in a short period between adjacent cells (i.e., ping-pong handover), and handover to another cell after a short-time stay (i.e., rapid handover). A handover is a change of connection of a mobile station communicating with a serving cell to a neighbour cell. In the case of UTRAN (UMTS Terrestrial Radio Access Network), for example, “communicating” means the state (CELL_DCH state) where an individual channel (i.e., Dedicated Channel (DCH)) is satisfied between a base station and a mobile station or between a radio network controller (RNC) and a mobile station. Further, in the case of E-UTRAN (Evolved UTRAN), “communicating” corresponds to RRC_CONNECTED state where data is transmitted and received between a base station and a mobile station using downlink and uplink shared channels (i.e., Physical Downlink Shared Channel (PDSCH) and Physical Downlink Shared Channel (PUSCH)).
In UTRAN and E-UTRAN, a handover control entity of a serving cell, such as a base station or RNC that manages the serving cell, instructs a mobile station to send a measurement report when a predetermined HO initiation condition is satisfied. One example of a HO initiation condition is degradation of radio quality of a serving cell. Note that, with regard to measurements relating to HO in E-UTRAN, the following five events A1 to A5 are specified in the 3GPP technical specification TS 36.331. Generally, either one of the event A3 or A5 is used as a HO initiation condition.
Event A1: Serving becomes better than absolute threshold;
Event A2: Serving becomes worse than absolute threshold;
Event A3: Neighbour becomes amount of offset better than serving;
Event A4: Neighbour becomes better than absolute threshold;
Event A5: Serving becomes worse than absolute threshold1 AND Neighbour becomes better than another absolute threshold2.
A measurement report generated by a mobile station contains a measurement result of radio quality of a neighbour cell (or neighbour cells) adjacent to the serving cell. In response to receiving a measurement report from a mobile station, a handover control entity of a serving cell determines a handover destination cell (i.e., target cell) based on the measurement report and initiates signaling for handover with a handover control entity of the target cell.
Accordingly, transmission of a measurement report by a mobile station in UTRAN and E-UTRAN is considered as a HO initiation request and is used as a triggering operation to initiate handover. Therefore, in UTRAN and E-UTRAN, TTT is used to adjust a lag time from when a HO initiation condition is first satisfied to when a measurement report is actually transmitted. For example, a mobile station determines a handover (HO) initiation state based on that a period during which a HO initiation condition that radio quality of a neighbour cell is higher than radio quality of a peripheral cell continues to be satisfied becomes longer than TTT, and then transmits a measurement report as a HO initiation request. Because the duration of the HO initiation condition needs to be longer as TTT is longer, the handover initiation timing is delayed and handover becomes difficult to occur.
Note that, in this specification, a state that serves as a trigger to initiate handover of a mobile station is referred to as “HO initiation state”. A “HO initiation state” is determined based on that a period during which a “HO initiation condition” continues to be satisfied becomes longer than TTT. When the “HO initiation state” is met, handover of a mobile station is initiated.
Further, in cellular radio communications, it is known to employ cell arrangement where a large-scale cell (e.g., a macrocell) and a small-scale cell (e.g., a microcell, a picocell, a femtocell) are laid to overlap each other as one solution to meet a demand for sufficient communication capacity and a demand for sufficient coverage. The overlap of the large-scale cell and the small-scale cell includes not only complete overlap in which the small-scale cell is completely included in the large-scale cell but also partial overlap in which a part of coverage of the large-scale cell and a part of coverage of the small-scale cell overlap. Such cell arrangement is called Hierarchical Cell Structure (HCS) or Heterogeneous Network (HetNet).
Non Patent Literature 1 shows a result of study about reduction of handover failures in HCS (HetNet). To be more specific, Non Patent Literature 1 presents a simulation result indicating that handover-failure rate, from a macrocell to a picocell, is significantly higher than handover-failure rate between macrocells particularly when a mobile station is moving at high speed and TTT (Time to Trigger) is long. Thus, Non Patent Literature 1 proposes that it is necessary to set a different TTT for handover to a picocell than that for handover to a macrocell.
Further, Patent Literature 1 discloses a technique to reduce handovers, of mobile stations moving at high speed, from a large-scale cell (for example, a macrocell) to a small-scale cell (for example, a microcell). To be specific, Patent Literature 2 discloses a technique that configures downlink coverage of a microcell to be smaller than uplink coverage thereof in HCS (HetNet). Patent Literature 1 discloses three different ways as specific methods to achieve reduction of the downlink coverage: (1) reducing pilot transmission power of a microcell, (2) adjusting a tilt angle of a downlink antenna of a microcell base station and (3) virtually reducing pilot received power of a microcell used for determination of handover by giving offset. According to the technique disclosed in Patent Literature 1, it is expected to reduce handover from a macrocell to a microcell.
Furthermore, in cellular radio communications, handover optimization is known as one technique related to SON (Self Organizing Network). Patent Literature 2 discloses a technique that reduces handover failures (i.e., too late handover, too early handover) by dynamically adjusting HO parameters including TTT based on measurement reports from mobile stations. To be specific, Patent Literature 2 discloses a technique that shortens TTT for reduction of too late handovers and lengthens TTT for reduction of too early handovers.